1. Field of the Invention
The present invention relates to in-vivo human subject medical imaging systems using Nuclear Magnetic Resonance (NMR) three-dimensional imaging and more particularly to the correction of distortion errors in NMR systems.
2. Related Art
In Nuclear Magnetic Resonance (NMR) or (MR) a strong magnetic field is applied across a subject and precisely pulsed radio frequency radiation waves are also applied. An atomic nucleus, for example, an atom of a portion of the human brain, as a result of its spin, has slightly different energy values. The nucleus absorbs the radio-frequency radiation and changes its energy value. That change is detected by a coil.
In one type of NMR system a steady (static) uniform magnetic field is applied in one plane and a linear gradient magnetic field is applied perpendicular (orthogonal) to the steady field. The gradient field is turned (rotated about an axis. Three-dimensional data is obtained, processed by a computer system and used to produce a set of images representing slices of the body taken in various directions.
NMR imaging permits patients to be diagnosed without an invasive surgical procedure such as biopsy. It does not use X-rays, as in a CT scan, which may be harmful and yet permits a series of images to be taken over time.
While Magnetic Resonance (MR) images are invaluable because of their high degree of anatomical differentiation and their lack of ionizing radiation, they contain an inherent distortion which can exceed ten percent. The distortion arises from a number of sources including magnetic field inhomogeneities and patient-specific factors.
A series of articles has examined various aspects of the distortion problem. The articles are cited herein by their first author and year and the full citation is given at the end in the section entitled "Literature Cited". These articles, and the patents cited below, are incorporated by reference at the place of their citation. The articles relating to the distortion problem are: Lai, 1982; Crooks, 1984; Bendel, 1985; Pykett, 1983; Manassen, 1985; Sekihara et al, 1984; Sekihara et al, 1985b; Shenberg & Macovsky, 1985a; Feinberg, 1985; Hutchison et al., 1984. Although the existence of this distortion has not been well documented by MR device manufacturers, without distortion compensation, quantitative MR measurements, regarding position, length, area and volume, are likely to be erroneous.
The distortion can be classified as subject dependent or subject independent Subject independent distortion arises mainly from magnetic field inhomogeneity and calibration error which remains the same from patient to patient. It can therefore be measured and corrected from images of a phantom of known dimensions. Subject-dependent distortion changes with the chemical makeup and geometry of the subject being scanned, and is particularly difficult to detect and remove.
In U.S. Pat. No. 4,425,547 entitled "Nuclear Magnetic Resonance Apparatus Having Means For Compensating a Projecting Signal", a pair of standard signal sources of a specified atomic nucleus is positioned in a NMR device, around a body to compensate a projection signal.
U.S. Pat. No. 4,672,320 entitled "Imaging Apparatus and Method Using Nuclear Magnetic Resonance" describes a direct Fourier imaging system providing compensation for distortion caused by (i) changes in intensity of the static magnetic field from a predetermined standard and (ii) changes in intensity of the gradient magnetic field from a different predetermined standard. The system uses a computer-based system for such compensation and a small probe having a signal detecting coil.
U.S. Pat. No. 4,300,096 entitled "Imaging Systems", in a NMR device, the magnetic fields at a plurality of positions in the plane of a slice are measured to provide error signals used to adjust the gradient and orthogonal magnetic fields.